ML20008A162
| ML20008A162 | |
| Person / Time | |
|---|---|
| Site: | Davis Besse  |
| Issue date: | 07/31/1997 |
| From: | William Burton NRC |
| To: | |
| References | |
| NUDOCS 9708290259 | |
| Download: ML20008A162 (5) | |
Text
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CEAB EVENT TRACKIN3 CHEET f
~ oxkE*a$$fN Assigned To & Assigned Date >= 07/10/97 & Assigned Date <= 07/10/97 d
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Unit:
1 Engineer:
BURt0N W.
Plants DAv!S BESSE Evcc.t:
07/10/97 Norning Report:
Briefing:
50.72i O
LERf:
050000009700000 PN#:
CtM r Notification:
CONVERS: HANSON/SKEEN ON 7/10 Gyctems Component:
QPERATING NODE SIGNIFICANCE h1-Operation A - Reactor Protection System z - Startup B - Safety-Related Cooling System 3 - Hot Stantby C - Fuel Cladding 4 - Hot Shutdown D - Reactor Coolant Pressure Boundary 5 - Cold Shutdown E - Containment 6 - Refueling F - Plant Power 7 - Other h-UnexpectedPlantPerformance H - Other:
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i CAUSJ EVENT TYPE M
(L,e 1 - Equipment Failure SIG - Significant Event (,D Oz - 0 isa or in tattation error
@-eventofintere=t Operating Error g 1BD - To Be Determined 4 - Maintenance Error OTH - Other 5 - External 6 - Other P
POTENTIAL AO:
Criterion:
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Pr: pored By:
BURTON W.
Apprsved:
2 7
Section Leader A. Chaffee Branch Chief 4
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C etu[re EVENTS ASSESSMENT PANEL First Screening: )
significance
Description:
NON-CONSERVATIVE ACCIDENT ANALYS15-LIMITING BREAK SIZE WRONG j
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420 FILE CEU5! 00py lETS-02 9708290259 970731 Py Page:1 07/31/97 e
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G:\\ FORMS \\ETS.RV1 l
psm+on sen PANEL -- X YES
. NO l
l ASSIGNMENT DATE:
July 10.1997 ASSIGNED T0:
BURTON PLANT & UNIT:
DAVIS BESSE EVENT DATE:
July 10. 1997 50.72 REPORT NO:
MR N0:
OTHER SOURCE REPORT:
CONVERS: HANSON SKEEN ON 7/10 RELATED REPORTS:
SPECIALTY CODE:
EVENT / CONDITION
SUMMARY
NON-CONSERVATIVE ACCIDENT ANALYSIS-LIMITING CIRC WATER BREAK SIZE WRONG SPECIFIC FOLLOW-UP ASSIGNMENT l
DETERMINE DETAILS. EVALUATE SAFETY SIGNIFICANCE AND GLWERIC IMPLICATIONS.
IN ADDITION. ADDRESS THE FOLLOWING SPECIFIC CONCERNS:
BRIEF ME ON DETAILS: DETERMINE GENERIC SIGNIFICANCE AND INVESTIGATIVF STATUS i
PREPARE TO BRIEF:. YES
. NO TARGET CLOSE0VT SCHEDULE:
INITIAL SCREENING BY PANEL REGULATORY ASSESSMENT:
In October 1995, the licensee identified that as a result of a newly postulated pipebreak scenario in the circulating water (CW) system, significant flooding of the turbine building could occur that would cause a loss of all main feedwater as well as a loss of auxiliary feedwater. This event would place the plant in a condition beyond its current design basis and require the use of primary side feed-and-bleed operation to cool the reactor core. Specifically, a CW pipe ru,rture causing a break flow of 300.000 GPM to the main condenser pit could flooo the main feedpumps. condensate pumps, heater drain pumps and the motor driven feedpump (all are located in condenser pit). After the condenser ait fills, water flows out onto the 585 ft elevation (ground level) of turbine Juilding. Water levels continue to rise, peaking at 27 inches above the 585 ft elev. At this point. water begins to l
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- e, flood the CW pumphouse located in turbine building adjacent to condenser pit.
Approximately 5.5 minutes after the pipe ru)ture. the water level reaches the
. top of AFW ventilation intake curbs above tie AFW pum) rooms and the AFW pump rooms begin to flood. In addition. high voltage swittigear room A begins to take on water. A) proximately-30 seconds after the initiation of flooding in j
- the AFW rooms, t1e AFW system becomes inoperable.
After approximately 8 minute.s. the CW pumphouse floods to the elevation of the i
CW pump motors, tri Water levels l
begin to decrease. pping the CW pumps and terminating the flood.
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The original licensing basis for internally initiated flooding at Davis-Besse i
comprised only a double-ended rupture of the CW pi)ing at the expansion
-I bellows at the inlet to the main condenser. No otler flooding from a pipe l
break scenario was analyzed.
J In response to the expansion bellows ru)ture, the licensee installed (at
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initial plant licensing) pressure swittles on the discharge piping from each CW pump to trip the pump upon sensing a break large enough to cause a sufficiently low discharge pressure. Because of this design feature, the newly )ostulated most-limiting CW pipebreak is now a slightly smaller break than tlat required to initiate the low pressure switches such that the CW pumps continue to pump and supply forced flow through the break.
Potential corrective ' actions were develo)ed in late 1995 and an outside contractor was retained to perform a pro)abilistic safety assessment-(PSA).
The results of the PSA were provided to the site in May 1996 and indicated '
that the timeline from initial pipebreak to a loss of all feedwater (including i
AFW) was approximately six minutes in the worst case.
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In addition, the analysis indicated that operators would not have time to i
L respond to the event prior to the loss of all feedwater.
Inadequate time was
.available for operators to:
- 1) assess and identify the flooding condition. 2) reference the appropriate abnormal procedure, and 3) to take actions to secure I
L the pumps (the pumps are interlocked with the discharge valves so that the
- pumps cannot be trip)ed until.their associated discharge valves have been closed). As such, t1e PSA concluded that this one accident sequence increased the overall plant core damage frequency (CDF) by 25 percent (6.6E-5 to 8.2E-5).
The licensee is currently in the process of installing a series of level switches in the main condenser pit to provide a direct automatic trip of the t
CW pumps upon sensing the start of ~a flooding condition.
Interim measures l
implemented in the meantime include:
(1) requiring an operator to be t
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stationed at the condenser pit when CW pumps are started / stopped:
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l (2) conducting an inspection of the CW expansion joints: and (^s) improving the material condition of the CW discharge valves by modifying the valve operators to better assure their capability to close if-needed (Mate: torque switch settings have been increased on two of the four valves to date).
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In addition, following discussions with the NRC morc complete interim i
-compensatory measures were impleented to give operators more time to respond to the event, as well as to add procedural guidance and training for the operators to aid them in their response actions. Specifically, the turbine j
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building rollup door is now to be maintained at least 3 feet open to provide an additional flowpath for the water to exit the building (which results in increasing the time to AFW flooding to 12 minutes), giving operators l
additional time to respond.
i The operations abnormal procedure for flooding has also been revised to require operators take "immediate actions" (those they have to memorize versus having to reference a procedure) to more quickly secure the CW pumps when
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symptoms warrant. The operators have now been trained on the procedure change as well as the PSA conclusions and expected flooding times for this accident.
The NRC is currently monitoring the licensee's progress in installing the condenser pit level switches. The licensee anticipates the modification will i
be ready for final connection and testing by the end of the month. They are also evaluating whether they can aerform the final work and safely conduct post-modification testing (PMT) w1ile at power.
If so, they intend to complete the work immediately thereafter.
If they cannot complete final installation and PMT while at power, the installation will be completed at the next scheduled or forced shutdown.
The NRC is also evaluating the validity of certain conclusions contained in the PSA. Specifically. the PSA indicated that slightly more than inches of i
water would flood high voltage switchgear room A, but no failure of equipment in the room would result.
Preliminary NRC review indicates that some of the switchgear could be affected due to a loss of DC control power to the
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breakers. Of particular concern is makeup (charging) pump A which is powered from switchgear room A.
The E0Ps direct use of both makeup (charging) pumps j
u to assure adequate feed-and-bleed operation.
If makeup pump A was unavailable, the probability of core damage could be increased beyond the 25 l
percent concluded in the current PSA.
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This postulated event has generic implications in that other licensees may not have fully considered the effects on safety-related equipment of circulating l
water pipe breaks which do not result in system pressures decreasing to the low pressure isolation setpoirts. 'It is recommended that a generic letter be
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prepared which requests licensees to review this scenario and take appropriate corrective actions. Because of the time to prepare generic letters. it is also recommended that an information notice be prepared discussing this event.
Finally it is recommended that this event be classified as an Event of 1
Interest.
PM: Allen Hansen - 415-1390 y
SPLB: Bill LeFave - 415-3285 l
i EVENT / CONDITION SAFETY SIGNIFICANCE: _ OTH _ E01 _ S..
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REMAINING OR ADDITIONAL FOLLOW-UP ITEMS:
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CLOSE00T TEXT
- REGULATORY ASSESSMENT:
(Abstract of Closecut/ Findings) l i
i CLOSE0UT/ FINDINGS:
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5 FINAL PANEL ASSESSMENT:
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-The Events Assessment Panel reviewed the package on August 5.1997 and
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concluded that this )ostulated event is an Event of Interest and an j
information notice s1ould be issued. The IN should include consideration of other flooding sources other than Circulating Water that may be present.
i EVENT / CONDITION SAFETY SIGNIFICANCE: _ OTH z E01 _ SIG._ AO BASIS: _ RISK _ _ PROGRAMMATIC ~ _ MARGIN _ N/A t
GENERIC FOLLOW-UP ACTION RECOMMENDED: 3 YES _ NO _ PRIORITY (1.2.3.4)2 l
3 IN
_ BUL
_ GL
_ NUREG
_ REG GUIDE l
i FILE: G:\\WFB\\DB_CW l
f BRIEF #
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1 Priority endnots;:-
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l PKiUKiH 1; jffmer, te assignment of resDurces (e.g., for highly risk-significant safety concerns)
PRIORITY 2: hear.gers action (e.g.. significant safety issues not marranting imediate action)
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PRIORITY 3: Lcmg,ers action (e.g. 1ssues of nederate to icw safety significance)
-l PRIORITY 4: Resource dependent action (e.g.. items that can be deferred) j i
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